684 ON CONCRETIONS, SPICULES, ETC. [ch. 



freedom which they undergo on entering one of these boundary 

 regions. And the same phenomenon is turned to account in the 

 various "separation-processes" in which metalHc particles are caught 

 up in the interstices of a froth, that is to say in the walls of the 

 foam-cells or Schaumkammern* . 



It is by a combination of these two principles, chemical adsorp- 

 tion on the one hand and physical quasi-adsorption or concentration 

 of grosser particles on the other, that I conceive the substance of 

 the sponge-spicule to be concentrated and aggregated at the cell 

 boundaries; and the forms of the triradiate and tetractinellid 

 spicules are in precise conformity with this hypothesis. A few 

 general matters, and a few particular cases, remain to be considered. 

 It matters httle or not at all for the phenomenon in question, what 

 is the histological nature or "grade" of the vesicular structures 

 on which it depends. In some cases (apart from sponges), they 

 may be no more than httle alveoh of an intracellular protoplasmic 

 network, and this would seem to be the case at least in the protozoan 

 Entosolenia aspera, within the vesicular protoplasm of whose single 

 cell Mobius has described tiny spicules in the shape of little tetra- 

 hedra with sunken or concave sides. It is probably the case also 

 in the small beginnings of Echinoderm spicules, which are likewise 

 intracellular and are of similar shape. Among the sponges we have 

 many varying conditions. In some cases there is reason to believe 

 that the spicule is formed at the boundaries of true cells or histo- 

 logical units ; but in the case of the larger triradiate or tetractineUid 

 spicules they far surpass in size the actual "cells." We find them 

 lying, regularly and symmetrically arranged, between the "pore- 

 canals" or "ciliated chambers," and it is in conformity with the 

 shape and arrangement of these large rounded or spheroidal struc- 

 tures that their shape is assumed. 



Again, it is not at variance with our hypothesis to find that, in 

 the adult sponge, the larger spicules may greatly outgrow the 

 bounds not only of actual cells but also of the ciliated chambers, 

 and may even appear to project freely from the surface of the 

 sponge. For we have already seen that the spicule is capable of 



* The crystalline composition of iron was recognised by Hooke in the Micro- 

 graphia (1665); and the cellular or polyhedral structure of the raetal was clearly 

 recognised by Reaumur, in his Art de convertir le fer forge en acier, 1722. 



